1. Field of the Art
The present invention relates in general to a ceramic envelope device for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge), and more particularly to electrically conducting end caps or closure discs which close the opposite ends of a translucent ceramic tube which cooperates with the end caps to form a gas-tight envelope incorporated in a HID lamp.
2. Related Art Statement
In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conducting discs are known as end caps to close the opposite open ends of the translucent ceramic tube. Examples of such closure end caps are illustrated in U.S. Pat. Nos. 4,155,757 and 4,155,758. Such end caps are formed of an electrically conducting cermet obtained by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. These electrically conducting cermet end caps support a pair of tungsten electrodes at their inner surfaces in the interior of the ceramic envelope so that the electrodes protrude from the inner surfaces of the end caps toward each other, i.e., longitudinally inward of the translucent ceramic tube. In the meantime, electrical contact rods or lead rods are connected or fixed to the outer surfaces of the cermet end caps with suitable methods. Electric power is applied to the pair of opposed tungsten electrodes through the contact rods and through the cermet end caps. Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because they eliminate the need of using expensive metallic niobium. It is further recognized that such cermet end caps have also been used advantageously for so-called metal halide lamps which employ translucent ceramic tubes charged with a suitable metal halide together with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides.
However, such a HID lamp with a translucent ceramic tube closed by cermet end caps may suffer a problem generally known as "arc-back" phenomenon wherein an arc will take place between the electrodes and the corresponding cermet end caps, rather than between the opposed electrodes, when the HID lamp is initially turned on. This arc-back phenomenon causes the cermet end caps to crack, thereby causing the translucent ceramic tube to leak. In addition, the "arc-back" phenomenon gives rise to vaporization and scattering of the refractory metal component of the cermet, and consequent deposition thereof on the inner surface of the ceramic tube, which results in blackening of the wall of the translucent ceramic tube, thereby reducing the degree of its luminous flux.
It is also recognized that supersaturated metal halide in the ceramic tube of a metal halide lamp may be condensed at a cold spot in the tube, e.g., at the lower end portion of the ceramic tube disposed vertically when the lamp is used in its upright position, whereby the cermet end caps closing the end portions of the tube are subject to corrosion due to the liquid phase of condensed metal halide which is currently suffering thermal conduction from the heated electrodes. Severe corrosion of the cement end caps may prevent the end caps from supporting the electrodes in their upright posture.